The present invention relates to a belt-type continuously variable transmission (CVT), and more particularly to a belt-type CVT whose driving pulley and driven pulley are formed into a two-piston structure.
U.S. Pat. No. 5,295,915 discloses a belt-type CVT whose driving pulley and driven pulley are formed into a two-piston structure.
However, the belt-type CVT of U.S. Pat. No. 5,295,915 has a problem that it is necessary that a torque cam mechanism for controlling a belt clamping force is disposed at an end of an input shaft. This arrangement increases an axial dimension of the belt-type CVT and generates a considerable loss of a hydraulic pump.
It is therefore an object of the present invention to provide an improved belt-type CVT which ensures a high pump-efficiency without increasing an axial dimension of the CVT.
An aspect of the present invention resides in a belt-type continuously variable transmission which comprises a driving pulley having a driving-side transmission piston chamber and a driving-side clamp piston chamber; a driven pulley having a driven-side transmission piston chamber and a driven-side clamp piston chamber; an endless belt wound around the driving pulley and the driven pulley so as to transmit a driving force from the driving pulley to the driven pulley; and a hydraulic circuit controlling effective pulley diameters of the driving pulley and the driven pulley relative to the belt according to vehicle condition indicative information to continuously vary a transmission ratio of the belt-type continuously variable transmission; wherein at least one of first and second relationships is satisfied, the first relationship being that an effective cross sectional area of the driving-side transmission piston chamber is equal to an effective cross sectional area of the driven-side transmission piston chamber, and the second relationship being that an effective cross sectional area of the driving-side clamp piston chamber is equal to an effective cross sectional area of the driven-side clamp piston chamber; wherein one of third and fourth relationships is satisfied, the third relationship being that a common passage of the hydraulic circuit fluidly communicates the driving-side transmission piston chamber and the driven-side transmission piston chamber, the fourth relationship being that the common passage fluidly communicates the driving-side clamp piston chamber and the driven-side clamp piston chamber.